In coating systems of the abovementioned type known from the market, the application device, which in painting processes may involve, for example a high-speed rotating atomizer, is supplied with a coating material, which is applied to the object to be coated.
Apart from paints, other coating materials can also be applied using various application devices, for example preservatives such as wax, individual components of multi-component adhesives, or possibly even high viscosity substances such as sealing materials.
By way of example, in the case of a painting system a coating material change device, that is to say, therefore, a colour changer, is used, if in normal operation it frequently occurs that for the coating of an object another paint has to be used that differs from that with which a previous object was painted.
In the event of a colour change the channels and lines carrying the material must have the previously used paint cleaned out from them, to which end a flushing agent is passed through the corresponding channels and lines. In order to keep the paint losses and quantities of flushing agent required as low as possible, what is known as the pigging technique is often used in which the coating materials or flushing agent are pushed through the channels and lines with the help of pigs. In doing so the pig is moved back and forth between two pigging stations, one of which is disposed in the vicinity of the application device and the other in the vicinity of the change device.
It has become established, in particular, that coating material that has not been applied, and which remains in the lines to the application device, is fed back into its material source, in order to minimise the paint losses.
Here, however, it is of major importance that no foreign materials reach the material source. In the present case air is also understood to be such a foreign material. Known colour changers are supplied by what are known as ring circuits. In this case in particular air bubbles must be prevented from entering a ring circuit since otherwise proper metering-in of specified quantities of material from this ring circuit is hampered.